1. Field of the Invention
This invention concerns a method of bonding a heat expandable fluoroplastic material to a metal substrate. More particularly, this invention concerns a method of bonding a heat-expandable fluoroplastic film under heat and pressure to a preselected surface of a metal substrate via a thermosettable solvent-activatable adhesive composition.
2. Description of Prior Art
Fluoroplastics are a class of parrafinic polymers which have some or all of the hydrogen replaced by fluorine. These materials are defined by the American Society for Testing and Materials (ASTM) as plastics based upon polymers from monomers containing one or more atoms of fluorine or copolymers of such monomers with other monomers, the fluorine-containing monomer(s) being present in the greatest amount by mass (ASTM D833). Fluoroplastics are made by free radical initiated polymerization or copolymerization of the monomers. Fluorocarbon plastics, those made from perfluoro monomers, include polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP), and perfluoroalkoxy resin (PFA). Other fluoroplastics include ethylenetetrafluroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE) and polyvinyl fluoride (PVF).
Fluoroplastics exhibit performance characteristics such as: resistance to harsh chemicals, a wide range of working temperatures (-300.degree. F. to 500.degree. F.), outstanding electrical insulation properties, low dielectric constants and high resistivity, and very good flame resistance. Fluoroplastics also have a low co-efficient of friction giving them non-adhesive and self-lubricating surface qualities.
These non-adhesive properties are a disadvantage when it is necessary to bond fluoroplastics to other materials. In such cases, the surface of the fluoroplastic to be bonded must be treated or etched so that it can accept an adhesive. Fluoroplastics can be effectively etched with a solution of metallic sodium in anhydrous ammonia. The fluoroplastic is either dipped in the etchant solution, or, if it is not to be etched all over, etchant is applied only to the surfaces to be bonded while the other surfaces are protected from the etchant. In the case of fluoroplastic tapes or sheets, in general, only one side is etched.
Fluoroplastic tapes or sheets have been etched and bonded via an adhesive film layer to a metal substrate which itself had been treated, e.g., sand blasted. In one technique, the fluoroplastic is coated with a liquid adhesive using a brush or a spatula. Disadvantages of this technique are that: (1) it is difficult to obtain a uniform thickness of liquid adhesive; (2) the liquid adhesive tends to spread over the edges of the fluoroplastic film during the bonding step; (3) the lubricity provided by the liquid adhesive tends to result in the aligned parts sliding out of alignment during the bonding step; and (4) the liquid adhesive must be applied just prior to the bonding step, i.e., before the adhesive loses its stickiness upon extended exposure to air.
In another technique, the fluoroplastic film and an adhesive film are separately stamped or cut out into pieces having preselected shapes. With different size tools, the fluoroplastic and adhesive pieces are aligned and then are bonded together under heat and pressure. Disadvantages of this technique are that: (1) because the adhesive piece will spread or flow when it is bonded to the fluoroplastic, different size pieces must be cut out in separate steps; and (2) it is difficult to maintain alignment of the metal fluoroplastic and adhesive pieces while bonding them together by application of heat and pressure.